Electrolytic stripping composition

ABSTRACT

A METHOD AND COMPOSITION FOR ELECTROCHEMICALLY REMOVING METALLIC LAYERS FROM A WORK-PIECE IN WHICH THE WORKPIECE IS DISPOSED IN AN ELECTROLYTIC BATH OF AN AQUEOUS SOLUTION OF AMMONIUM NITRATE, TARTARIC ACID AND SODIUM HYDROXIDE.

United States Patent O US. Cl. 204-14 1 Claim ABSTRACT OF THE DISCLOSURE A method and composition for electrochemically removing metallic layers from a Work-piece in which the workpiece is disposed in an electrolytic bath of an aqueous solution of ammonium nitrate, tartaric acid and sodium hydroxide.

This application is a division of our co-pendinig application Ser. No. 675,338, filed Oct. 16, 1967 and now Pat. No. 3,492,210.

BACKGROUND OF THE INVENTION In the metal plating of diiferent articles, the articles are normally carried through plating baths on carriers formed from a ferrous base metal and mounted on a conveyor. Consequently, the metal being plated onto the articles is also plated onto the carriers with the result that after the carriers have been in use for a period of time they are coated with layers of the metal or metals being deposited by the plating baths through which they have passed. Our invention is directed to a process and stripping composition which will quickly remove such coating layers without etching the ferrous base material of the carrier and which avoids the use of any of the toxic cyanides.

SUMMARY OF THE INVENTION In accordance with the invention, a stripping bath for removing brass, chrome, nickel, cadmium, zinc, or the like from a ferrous based work-piece is prepared, said bath consisting of an aqueous solution of ammonium nitrate, tartaric acid, and sodium hydroxide. The bath is maintained at an elevated temperature while the Workpiece which is to have the metal removed therefrom is passed therethrough. The workpiece immersed in the bath and said work-piece and an electrically conductive member in contact with the bath are made the anode and cathode, respectively, of an electrolytic circuit through which a high amperage current is passed with the result that the metal on the work-piece is electrolytically stripped therefrom and forms a sludge in the bath and/or a coating on the electrically conductive member.

DETAILED DESCRIPTION Our invention is concerned with a stripping process and agent for removing layers of brass, chrome, nickel, cadmium, zinc, or the like from ferrous based metals. It therefore finds particular utility in removing such metals from carriers on conveyors, which carriers may have been previously used to carry various parts or items through metal plating baths.

Our stripper is comprised of an aqueous solution of ammonium nitrate, tartaric acid, and sodium hydroxide. The ammonium nitrate is present in an amount ranging from 1.0 oz. to 80.0 oz. per gallon of solution, the tartaric 3,649,491 Patented Mar. 14, 1972 ice acid in an amount ranging from .01 oz. to .80 oz. per gallon of solution, and the sodium hydroxide in an amount ranging from .037 oz. to .36 oz. per gallon of solution. The ammonium nitrate serves as the Stripping agent for the coating layers to be removed, and consequently, as the amount of ammonium nitrate in the solution is decreased, the rate of the stripping action of the solution is reduced. The tartaric acid acts as a buffer for the workpiece, and thus, it it is present in the solution in amounts less than .01 oz. per gallon, etching of the workpiece will occur. Amounts of ammonium nitrate and tartaric acid in the solution greater than 80.0 oz. per gallon and .80 oz. per gallon, respectively, show no improved results, and thus it is not economically sound to use greater amounts of these two materials. Desirably, the tartaric acid is present in the solution in an amount equal to one-hundreth the amount of ammonium nitrate in the solution. The sodium hydroxide neutralizes the solution into a pH range of from 6.8 to 8.0. If the pH of the solution drops below this range, etching of the work-piece will occur, and if the pH of the solution goes above this range, the rate of stripping action of the solution is substantially reduced. Thus, a typical example of a stripping solution which we have successfully employed consisted of an aqueous solution containing 24.0 oz. of ammonium nitrate, .24 oz. of tartaric acid, and .11 oz. of sodium hydroxide for each gallon of solution.

As previously indicated, the pH of the stripping solution should be maintained between 6.8 and 8.0. Under normal operation the pH of the solution should remain stable. However, if the pH falls below this range, it can be raised by the addition of sodium hydroxide, or if it rises above this range, it can be lowered by the addition of nitric acid.

In order to remove coatings of brass, chrome, nickel, cadmium, zinc, or the like from a ferrous based workpiece, such as a conveyor carrier, a bath of the stripper is prepared and placed in a tank where it is maintained at an elevated temperature, desirably in the range from F. to 165 F. We have found that at temperatures below this range the rate of the stripping action is substantially reduced, and that at temperatures above this range ammonia is evolved from the bath. The workpiece is placed in said bath, and it and an electrically conductive member in contact with the bath are connected to an electrical circuit to form an electrolytic cell with the work-piece constituting the anode and the electrically conductive member constituting the cathode. A current of from 100 to 200 amperes per square foot of Work-piece surface at a voltage of from 5 to 8 -volts is applied to circuit. If the amperage and voltage are below this range, we have found that the stripping rate will be substantially reduced, and if the amperage and voltage are above this range, etching of the work-piece will occur.

The metal of the coating being stripped is electrolytically stripped from the work-piece at the rate of about .10 mils per minute and becomes a sludge in the bath and a deposit on the cathode forming member. The bath life can be increased by periodic filatration or decanting to remove the sludge and by cleaning the cathode forming member.

A typical example of our invention is seen wherein a nickel and chrome plating was removed from a series of steel plates having a total surface area of about 1.5 square feet. A stripper bath was prepared using water as the diluent and containing 24.0 oz. of ammonium nitrate, .24 oz. of tartaric acid, and .11 oz. of sodium hydroxide for each gallon of the bath. The bath had a pH of about 7.1. The bath was placed in a steel tank and heated to a temperature of about F. The plates were placed in the tank and said plates and tank were connected to an electrical circuit with the plates forming the anode 3 and the tank the cathode. A current of 180 amperes at 6 volts was impressed on the circuit. In 5 minutes the nickel and chrome coating was reduced in thickness by .5 mil, the stripped nickel and chrome forming a sludge in the tank and a film on the tank walls.

Although our invention has been referred to herein as being used to strip metal coatings from ferrous based, conveyor carriers, it is to be understood, of course, that it can be used to strip coatings from any ferrous based work-piece.

We claim:

1. A stripping agent for an electrolytic bath for removing coatings of brass, chrome, nickel, cadium, zinc, or the like from a ferrous based workpiece comprising an aqueous solution containing from 1.0 oz. to 80.0 oz. of ammonium nitrate, from .037 oz. to .36 oz. of sodium. hydroxide, and from .01 oz. to .80 oz. of tartaric acid per gallon of solution, said solution having its pH adjusted into the range of from 6.8 to 8.0.

References Cited UNITED STATES PATENTS 5/1941 Day 204146 1/1952 Larsen 204-146 1/ 1932 McCullough 204-146 9/ 1964 Hendry 204146 6/ 1957 Schaefer 204-146 FOREIGN PATENTS 9/1967 Great Britain 204-145 9/1967 Great Britain 204-145 US. Cl. X.R. 

